Upon launch, even though our group did beat the game on Inferno, pre-nerf, it felt like an incomplete, bug-ridden game.
Not as good as Diablo 2.

But lately, after all the patches, and such, changes to the loot system, Paragon levels, and now that everyone starts from clean slate, since new levels and loot..
The game is turning out to be really good.

Still not sure if you should buy the expansion? Give me a few days to play, I'll report back.
You're gonna be few days behind, at that point!